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View Full Version : Swerving with Level Sidespin ? - Science Heavy

Fred Agnir
08-11-2005, 06:34 AM
Maybe this needed to be a private conversation, but I thought it was an important topic enough that if resolution came from it, it might answer or debunk the question.

We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail.

That being said, for the science guys... JAL, Bob Jewett, Dr. Dave, and others... consider this, and tell me what affect if any happens.

The standard Coriolis affect of a spinning object when it is subjected to a force is that there is a reactive force at some right angle. If you spin a ball in place and watch the axis of the ball, you will see the axis tilt when that ball hits a cushion or any object ball. This is consistent with Coriolis and any gyroscopic study (for spinning objects with no fixed point).

In a standard collision with standard english, this axis tilt will happen every time.

So, the question is, does the reactive force that creates this tilt have any affect on the overall path, initial path, initial angle, or anything? Does it have an effect on the resulting cushion rebound when compared to, say, a kick shot where there is no pre-collision with an object ball and therefore no axis tilt prior to hitting the cushion?

It seems to me that if the axis tilts, even though it's just a few degrees, that this tilt is a result of a force that's not in the same direction as the cue stick force, and therefore should be different than the force that describes using, say, low left-hand english with a level cue (which shouldn't swerve).

Fred &lt;~~~ could be all wet

SpiderMan
08-11-2005, 07:34 AM
That's a very good question, I've often wondered that myself. I believe we claim that the gyroscope represented by the side-spinning cueball is rolling forward with it's spin axis rotating in a vertical plane.

But, after a collision/carom off an object ball, this is no longer true. What is the post-collision behavior of the cueball, and how does it differ from either a sliding, rolling, or backspinning example?

If I were the mayor, I'd shine the "D" spotlight on a cloud about now /ccboard/images/graemlins/grin.gif

SpiderMan

Chopstick
08-11-2005, 07:48 AM
<blockquote><font class="small">Quote SpiderMan:</font><hr> That's a very good question, I've often wondered that myself. I believe we claim that the gyroscope represented by the side-spinning cueball is rolling forward with it's spin axis rotating in a vertical plane.

If I were the mayor, I'd shine the "D" spotlight on a cloud about now /ccboard/images/graemlins/grin.gif

SpiderMan <hr /></blockquote>

Maybe I misunderstand. I don't see how the spin axis could rotate in the vertical plane. When it rotated 45 degrees the radius of the spin would be perpendicular to the line of travel. I think as soon as you hit it, it does an axial tilt. I have it on tape somewhere. I'll go look.

This is a good point Fred. There are some shots I hit where I see the cue ball deflect and then the spin carries it back to line. Maybe it's just the way I hit them.

SpiderMan
08-11-2005, 08:20 AM
<blockquote><font class="small">Quote Chopstick:</font><hr> I don't see how the spin axis could rotate in the vertical plane. When it rotated 45 degrees the radius of the spin would be perpendicular to the line of travel. I think as soon as you hit it, it does an axial tilt. I have it on tape somewhere. I'll go look.<hr /></blockquote>

I think that's because what we can observe is a superposition of both forward roll and spin about a vertical axis (the axis which is now rotating forward).

If you watch or videotape the motion, a single "spot" on the ball surface will precess from one side to the other, as if the ball is somehow sliding forward while spinning "on a tilt".

SpiderMan

dr_dave
08-11-2005, 08:22 AM
Fred,

I will attempt to provide complete answers to all of your questions, but it will take a while. I need to combine all of the analyses from TP A.4 through TP A.8 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/index.html). I also want to add a "front-end" where the cue-tip-to-cue-ball contact point and cue stick elevation can be specified. Then I will be able to plot pre-impact and post-impact cue ball trajectories for almost any type of shot. I'll post an update in this thread when I'm done and have some examples to show. Unfortunately, there's the small matter of the semester starting for me on 9/22. My amount of free time is already starting to evaporate very quickly. But I like your questions and I also want complete answers. Luckily, I will be teaching a graduate course on advanced dynamics principles this semester, so I will be in a motion-analysis kind of mood.

Regards,
Dave

<blockquote><font class="small">Quote Fred Agnir:</font><hr> Maybe this needed to be a private conversation, but I thought it was an important topic enough that if resolution came from it, it might answer or debunk the question.

We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail.

That being said, for the science guys... JAL, Bob Jewett, Dr. Dave, and others... consider this, and tell me what affect if any happens.

The standard Coriolis affect of a spinning object when it is subjected to a force is that there is a reactive force at some right angle. If you spin a ball in place and watch the axis of the ball, you will see the axis tilt when that ball hits a cushion or any object ball. This is consistent with Coriolis and any gyroscopic study (for spinning objects with no fixed point).

In a standard collision with standard english, this axis tilt will happen every time.

So, the question is, does the reactive force that creates this tilt have any affect on the overall path, initial path, initial angle, or anything? Does it have an effect on the resulting cushion rebound when compared to, say, a kick shot where there is no pre-collision with an object ball and therefore no axis tilt prior to hitting the cushion?

It seems to me that if the axis tilts, even though it's just a few degrees, that this tilt is a result of a force that's not in the same direction as the cue stick force, and therefore should be different than the force that describes using, say, low left-hand english with a level cue (which shouldn't swerve).

Fred &lt;~~~ could be all wet <hr /></blockquote>

dr_dave
08-11-2005, 08:38 AM
<blockquote><font class="small">Quote SpiderMan:</font><hr>If I were the mayor, I'd shine the "D" spotlight on a cloud about now /ccboard/images/graemlins/grin.gif<hr /></blockquote>
Spiderman,

We don't get many clouds out here in Colorado, but I aim to swerve. /ccboard/images/graemlins/wink.gif I hope you meant me with your "D."

Dave

Cane
08-11-2005, 08:53 AM
The rotation of the axis of spin is called gyroscopic precession. It's just a necessary evil of spinning a ball, and it can be used to an advantage if it is understood.

As for swerve on level cue shots, well, what we have to keep in mind is that it's absolutely impossible to have an absolutely level cue. If you're hitting a tip left through core center, even if the butt of your cue is scraping the rail, you still have a few degrees of elevation. So, to a very small extent, the curving of the CB on non center hits is inevitable. Is it enough to make a difference on "most" shots? No, I don't think so, but it is enough to make a difference on some shots. When we, as poolplayers or instructors, talk about a level cue, we're talking about keeping it as close to level as possible without hitting the rail caps with your hand or cue, but an absolute level cue is just impossible. Knowing this, then we must know that anytime we hit the CB left or right of center through the core, then we are hitting it such that it would swerve, even if that swerve is negligible.

Later,
Bob

dr_dave
08-11-2005, 09:36 AM
<blockquote><font class="small">Quote Cane:</font><hr> The rotation of the axis of spin is called gyroscopic precession. It's just a necessary evil of spinning a ball, and it can be used to an advantage if it is understood.<hr /></blockquote>
Bob (and Fred),

FYI, I don't think "gyroscopic precession" is an appropriate description for the instantaneous change in the axis of rotation of the cue ball resulting from impact with an object ball. Gyroscopic precession refers to continual rotation (or wobble) of a spin axis due to a torque acting about an axis other than the spin axis (e.g., with a precessing top or gyroscope supported at one end). The change in cue ball rotation axis resulting from impact with an object ball is simply caused by changes in angular momentum resulting from the brief frictional impulse that occurs between the balls during impact. There are also no "Coriolis forces" at play during or after ball collisions. Coriolis forces apply only when there is translational motion on a rotating reference frame (e.g., winds of the surface of the earth turning CCW in the northern hemisphere, or sideways forces on your feet as you walk straight out on a rotating platform [e.g., a merry-go-round]).

<blockquote><font class="small">Quote Cane:</font><hr>As for swerve on level cue shots, well, what we have to keep in mind is that it's absolutely impossible to have an absolutely level cue. If you're hitting a tip left through core center, even if the butt of your cue is scraping the rail, you still have a few degrees of elevation. So, to a very small extent, the curving of the CB on non center hits is inevitable. Is it enough to make a difference on "most" shots? No, I don't think so, but it is enough to make a difference on some shots. When we, as poolplayers or instructors, talk about a level cue, we're talking about keeping it as close to level as possible without hitting the rail caps with your hand or cue, but an absolute level cue is just impossible. Knowing this, then we must know that anytime we hit the CB left or right of center through the core, then we are hitting it such that it would swerve, even if that swerve is negligible.<hr /></blockquote>
Well stated. FYI, I have illustrations and calculations for the minimum cue stick elevation required at TP A.3 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-3.pdf). Angles less than 2 degrees are possible, but that still ain't level.

Regards,
Dave

SpiderMan
08-11-2005, 11:09 AM
As a first analysis, could we boil it down to one enlightening case?

Assume the cueball could be struck "level" such that it starts out with sidespin only.

For this defining case, I'd also assume a perfectly elastic collision and frictionless ball-to-ball surfaces (but not frictionless ball-to-cloth). Why? Because I'd like to isolate this one effect. You can always modify the "reference-case" analysis for other variables later, should your initial analysis rule in the affirmative.

Can it be predicted that the CB follows a post-collision path that is different than if it had only forward roll?

Finally, if the answer is "yes" - using practical values of speed/spin ratio and ball-cloth friction, should the change in path be observable, or would we just be dreaming if we thought we saw it?

I specified zero ball-ball friction to leave out the discussion of spin "throwing" the balls away from the lines of common center and tangent, ie to initially consider only change in path AFTER the initial 90-degree departure.

Yes, Dr Dave, I was signaling you /ccboard/images/graemlins/grin.gif

SpiderMan

Billy_Bob
08-11-2005, 11:36 AM
From...
Mechanics of Billiards, and Analysis of Willie Hoppe's Stroke - Moore, A. D. - 1942.

"The Ball as a Gyroscope"
"When the billiard ball spins, it is a gyroscope: its spin axis tends to remain fixed on the same distant point in space. Next if a force is applied that tends to shift the axis, two things happen: the force is resisted, and there is precession of the axis. Precession means that instead of the axis shifting the way the force tries to shift it, it shifts instead at right angles to the force tendency and continues to do so as long as the shift force is applied."

Note: The above is a paper by... A. D. Moore, Professor of Electrical Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan - January 17, 1942.

The copy I have seems to be a copy of a copy of a copy of a typewritten paper. It is almost unreadable.

If someone could get a hold of a clean copy of the original from the University of Michigan Engineering Library, then post it on the internet, I think it would be a good thing (as Martha Stewart would say...). Then maybe I could read it!

(I bought mine from someone on Ebay.)

Fred Agnir
08-11-2005, 11:46 AM
<blockquote><font class="small">Quote Billy_Bob:</font><hr> From...
Mechanics of Billiards, and Analysis of Willie Hoppe's Stroke - Moore, A. D. - 1942.

"The Ball as a Gyroscope"
"When the billiard ball spins, it is a gyroscope: its spin axis tends to remain fixed on the same distant point in space. Next if a force is applied that tends to shift the axis, two things happen: the force is resisted, and there is precession of the axis. Precession means that instead of the axis shifting the way the force tries to shift it, it shifts instead at right angles to the force tendency and continues to do so as long as the shift force is applied."

Note: The above is a paper by... A. D. Moore, Professor of Electrical Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan - January 17, 1942.
<hr /></blockquote>Thanks Bill. I didn't want this to become a discussion on gyroscopic precession because it is a confusing term to laymen and engineers. I even deleted my post with hope to keep it off the table.

Your quotation is 100% in agreement with my understanding of gyroscopic motion. And I've done a fair amount of work with gyroscopes in the real world. Most people (and engineers) will only know precession as it pertains to tops.

How this version of precession affects (if at all) the path of a cueball is the question.

Fred

Billy_Bob
08-11-2005, 12:17 PM
I have "played" with gyroscopes in the past. But I guess you really need to hold the thing in your hands to understand how something "spinning" will resist change. Fun to play with though...

dr_dave
08-11-2005, 01:01 PM
<blockquote><font class="small">Quote SpiderMan:</font><hr> As a first analysis, could we boil it down to one enlightening case?<hr /></blockquote>
Yes.
<blockquote><font class="small">Quote SpiderMan:</font><hr>Assume the cueball could be struck "level" such that it starts out with sidespin only.<hr /></blockquote>
Ok.
<blockquote><font class="small">Quote SpiderMan:</font><hr>For this defining case, I'd also assume a perfectly elastic collision and frictionless ball-to-ball surfaces (but not frictionless ball-to-cloth). Why? Because I'd like to isolate this one effect. You can always modify the "reference-case" analysis for other variables later, should your initial analysis rule in the affirmative.<hr /></blockquote>
Sounds good to me.
<blockquote><font class="small">Quote SpiderMan:</font><hr>Can it be predicted that the CB follows a post-collision path that is different than if it had only forward roll?<hr /></blockquote>
With sidespin only, and no ball friction, the cue ball would head straight in the tangent line direction. With forward roll only, the cue ball curves in the 30 degree direction. See my May, 2005 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/may05.pdf) for illustrations and explanations.

Now, if you are comparing forward roll with no English to forward roll with English, and ball friction is neglected, the trajectories are the same. See the graph on page 6 of TP A.8 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-8.pdf). I just added a few graphs to the analysis to see the effects of different variables.
<blockquote><font class="small">Quote SpiderMan:</font><hr>Finally, if the answer is "yes" - using practical values of speed/spin ratio and ball-cloth friction, should the change in path be observable, or would we just be dreaming if we thought we saw it?<hr /></blockquote>
If you are comparing roll shots with and without English, under your ideal test conditions, there is no effect. FYI, the graphs in TP A.8 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-8.pdf) show various effects that might be of interest.
<blockquote><font class="small">Quote SpiderMan:</font><hr>I specified zero ball-ball friction to leave out the discussion of spin "throwing" the balls away from the lines of common center and tangent, ie to initially consider only change in path AFTER the initial 90-degree departure.<hr /></blockquote>
FYI, my April and May, 2005 instructional articles (http://www.engr.colostate.edu/~dga/pool/bd_articles/index.html) show how English, ball friction, and ball inelasticity affect various post-impact cue ball trajectories.
<blockquote><font class="small">Quote SpiderMan:</font><hr>Yes, Dr Dave, I was signaling you /ccboard/images/graemlins/grin.gif<hr /></blockquote>
Good job. It worked. How is Spiderman signaled in the comic strip, or is he?

Regards,
Dave

dr_dave
08-11-2005, 01:18 PM
I agree with Fred that we don't want this thread to become an argument over what "precession" means. If anbody really wants to know the accepted definition, they can refer to Webster's dictionary.

BillyBob, thanks for posting the quote anyway. It was interesting to read. I wish I could see the whole thing.

BTW, if people are interested in seeing demonstrations of gyroscopic motion and one type of "precession," I have a couple of interesting demos in the "Dynamics" area of my main (non pool) video demonstrations website (http://www.engr.colostate.edu/~dga/video_demos/index.html). Enjoy!

Regards,
Dave

<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote Billy_Bob:</font><hr> From...
Mechanics of Billiards, and Analysis of Willie Hoppe's Stroke - Moore, A. D. - 1942.

"The Ball as a Gyroscope"
"When the billiard ball spins, it is a gyroscope: its spin axis tends to remain fixed on the same distant point in space. Next if a force is applied that tends to shift the axis, two things happen: the force is resisted, and there is precession of the axis. Precession means that instead of the axis shifting the way the force tries to shift it, it shifts instead at right angles to the force tendency and continues to do so as long as the shift force is applied."

Note: The above is a paper by... A. D. Moore, Professor of Electrical Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan - January 17, 1942.
<hr /></blockquote>Thanks Bill. I didn't want this to become a discussion on gyroscopic precession because it is a confusing term to laymen and engineers. I even deleted my post with hope to keep it off the table.

Your quotation is 100% in agreement with my understanding of gyroscopic motion. And I've done a fair amount of work with gyroscopes in the real world. Most people (and engineers) will only know precession as it pertains to tops.

How this version of precession affects (if at all) the path of a cueball is the question.

Fred <hr /></blockquote>

SpiderMan
08-11-2005, 02:48 PM

I didn't think it would have an effect, but I lacked the ability to produce the modeling. It's been almost 30 years since I sat in dynamics class.

I arrived at the conclusion by superposition of two easier solutions: We are already aware of the parabolic post-collision path followed in the case of roll/no-spin, thanks to analyses by yourself and others.

Then I separately considered the spin/no-roll case, where the ball slides into the collision with nothing but sidespin. In that case, the spin axis never tilts (except for post-collision forward roll), allowing me to conclude that there is no path distorion.

Superposing these two, I decided that the combination should be identical to the roll/no-spin case.

BTW, the mayor used a spotlight device, which projected an image of a bat onto (ever-present when needed) clouds, to summon Batman /ccboard/images/graemlins/grin.gif

SpiderMan

SpiderMan
08-11-2005, 02:52 PM
<blockquote><font class="small">Quote Billy_Bob:</font><hr> I have "played" with gyroscopes in the past. But I guess you really need to hold the thing in your hands to understand how something "spinning" will resist change. Fun to play with though... <hr /></blockquote>

I once saw a video (sort of like "Candid Camera") where a powerful battery-operated gyroscope was concealed in a briefcase and left unattended. Anyone walking past could lift it by the handle and continue on their way, but as soon as they tried to turn, it would twist itself out of their grip.

SpiderMan

dr_dave
08-11-2005, 02:54 PM
<blockquote><font class="small">Quote SpiderMan:</font><hr> Thanks for the quick reply.

I didn't think it would have an effect, but I lacked the ability to produce the modeling. It's been almost 30 years since I sat in dynamics class.

I arrived at the conclusion by superposition of two easier solutions: We are already aware of the parabolic post-collision path followed in the case of roll/no-spin, thanks to analyses by yourself and others.

Then I separately considered the spin/no-roll case, where the ball slides into the collision with nothing but sidespin. In that case, the spin axis never tilts (except for post-collision forward roll), allowing me to conclude that there is no path distorion.

Superposing these two, I decided that the combination should be identical to the roll/no-spin case.

Was this a valid path to the same conclusion as your analysis, or just a coincidence?<hr /></blockquote>
Superposition can be dangerous at times, but it worked in this case.

<blockquote><font class="small">Quote SpiderMan:</font><hr>BTW, the mayor used a spotlight device, which projected an image of a bat onto (ever-present when needed) clouds, to summon Batman /ccboard/images/graemlins/grin.gif<hr /></blockquote>
I knew that. I was wondering how Spiderman (your namesake) was summoned, if he was.

Regards,
Dave

dr_dave
08-11-2005, 03:03 PM
<blockquote><font class="small">Quote Billy_Bob:</font><hr> I have "played" with gyroscopes in the past. But I guess you really need to hold the thing in your hands to understand how something "spinning" will resist change. Fun to play with though... <hr /></blockquote>
Another way to easily experience gyroscopic forces is to take the front wheel off a bike, hold both ends of the axle and spin up the wheel on the ground, then turn the spinning wheel left and right in the air. When doing so, you should feel a non-intuitive torque acting to and fro (about a vertical axis). This works better if you first replace the tire air with something heavy (e.g., lead shot). This demo can often be found in science musuems, where you usually sit in a swivel chair which turns for no apparent reason when you pivot the wheel. It's not magic ... it gyroscopic action.

Regards,
Dave

Jal
08-11-2005, 06:20 PM
<blockquote><font class="small">Quote Fred Agnir:</font><hr>...
We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail....<hr /></blockquote>

An interesting question Fred. I've been bogged down working on another problem related to throw and am not even taking the time to read the other responses right now. My apologies if I'm just repeating what has already been said in one form or another.

But your question got me excited because I actually made a "movie" showing the evolution of the cueball's spin axis, for various contact points and cue elevations, as it meanders down the table. It's fairly crude and I could hardly defend it now since I did it over a year ago and have forgotten the math details, but maybe I'll upload it to my ISP's server and provide a link just for fun.

I still think that sidespin will not affect the path after collision with an object ball. The torque produced by the friction will lay in the vertical plane in which the friction vector is embedded. In order to change the cueball's path the torque must have a component along the new direction of the cueball (tangent line). Pure sidepin will cause the torque to be straight up or down with no such component. To have some masse-like spin after the collision, it must have some draw/follow on it, but then the friction only reduces this a little bit.

I'll bet Dr. Dave will provide a more thorough analysis and maybe some exceptions to this rule. You really got me going Fred, but I'm just emerging from a state of complete and utter confusion over this other problem and am hesitant re-discombobulate myself right now. I am looking forward to reading what the others have said.

Jim

Chopstick
08-12-2005, 05:33 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote SpiderMan:</font><hr>Yes, Dr Dave, I was signaling you /ccboard/images/graemlins/grin.gif<hr /></blockquote>
Good job. It worked. How is Spiderman signaled in the comic strip, or is he?

Regards,
Dave <hr /></blockquote>

Spiderman has his spider sense so he doesn't need a signal. He knows. He usually found out something was up when his spider sense went off about two seconds before somebody dropped a bomb on him. And it is the police commissioner not the mayor who sends the signal. Come on guys we are trying to be precise here. /ccboard/images/graemlins/laugh.gif

Fred Agnir
08-12-2005, 06:02 AM
<blockquote><font class="small">Quote Jal:</font><hr> I still think that sidespin will not affect the path after collision with an object ball. The torque produced by the friction will lay in the vertical plane in which the friction vector is embedded. <hr /></blockquote>I think you're dead on with this. I was thinking a little more on this, and any axis tilt would be in a plane along the tangent, so the path would be unaltered. How about off a cushion?

Fred

dr_dave
08-12-2005, 08:10 AM
<blockquote><font class="small">Quote Jal:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>...
We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail....<hr /></blockquote>
I still think that sidespin will not affect the path after collision with an object ball. The torque produced by the friction will lay in the vertical plane in which the friction vector is embedded. In order to change the cueball's path the torque must have a component along the new direction of the cueball (tangent line). Pure sidepin will cause the torque to be straight up or down with no such component. To have some masse-like spin after the collision, it must have some draw/follow on it, but then the friction only reduces this a little bit.<hr /></blockquote>
Excellent summary! No disagreements here.
<blockquote><font class="small">Quote Jal:</font><hr>I'll bet Dr. Dave will provide a more thorough analysis and maybe some exceptions to this rule.<hr /></blockquote>
I will provide the math and physics details soon, with example trajectories for various cases, but I don't expect any exceptions to your well-stated summary.

Regards,
Dave

Qtec
08-12-2005, 08:54 AM

precession (http://videobillard.free.fr/Mecanique/precession.avi)

Qtec

Fran Crimi
08-12-2005, 09:05 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used.

Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail. <font color="blue"> Quick question: Are you referring here to the use of a level stroke or an elevated stroke as well?

Fran</font color>

Fred <hr /></blockquote>

dr_dave
08-12-2005, 09:21 AM
<blockquote><font class="small">Quote Qtec:</font><hr> One for your collection Dave.

precession (http://videobillard.free.fr/Mecanique/precession.avi)

Qtec <hr /></blockquote>
Rod,

I wasn't able to play this video on my computer. I tried Windows Media Player, Quicktime, Real, and DivX, and none of them worked. I even upgraded everything to the latest versions. Any advice?

Thanks,
Dave

Fred Agnir
08-12-2005, 09:21 AM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used.

Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail. <font color="blue"> Quick question: Are you referring here to the use of a level stroke or an elevated stroke as well?

Fran</font color>

Fred <hr /></blockquote> <hr /></blockquote>I was talking about "level," but I guess I needed to make sure I was talking about the cueball "curving" due to english.

Fred

Billy_Bob
08-12-2005, 09:27 AM
<blockquote><font class="small">Quote dr_dave:</font><hr>I wasn't able to play this video on my computer. I tried Windows Media Player, Quicktime, Real, and DivX, and none of them worked. I even upgraded everything to the latest versions. Any advice? <hr /></blockquote>

I've sometime had luck by right clicking on the link, then save target as, then save the file to my local computer. Then it will play OK. Works more often than not.

dr_dave
08-12-2005, 09:28 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used.

Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail. <font color="blue"> Quick question: Are you referring here to the use of a level stroke or an elevated stroke as well?

Fran</font color>

Fred <hr /></blockquote> <hr /></blockquote>I was talking about "level," but I guess I needed to make sure I was talking about the cueball "curving" due to english.<hr /></blockquote>
I usually assume that when somebody says "level stroke" they mean "minimal cue stick elevation" or "keep the cue stick as level as possible at impact with the cue ball," but I can see why there is confusion. By "level stroke," do you and Fran mean "drop your elbow during the stroke so the cue stick remains at the same angle during the stroke?" Please clarify.

Thanks,
Dave

SpiderMan
08-12-2005, 09:33 AM
<blockquote><font class="small">Quote dr_dave:</font><hr>I knew that. I was wondering how Spiderman (your namesake) was summoned, if he was.
Regards,
Dave <hr /></blockquote>

Back in the '60s, which was the last time I bought any comic books, SpiderMan was not summoned, he just "knew". He generally showed up, fought crime, and was later blamed for collateral damage /ccboard/images/graemlins/grin.gif

SpiderMan

dr_dave
08-12-2005, 09:37 AM
<blockquote><font class="small">Quote Billy_Bob:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr>I wasn't able to play this video on my computer. I tried Windows Media Player, Quicktime, Real, and DivX, and none of them worked. I even upgraded everything to the latest versions. Any advice? <hr /></blockquote>

I've sometime had luck by right clicking on the link, then save target as, then save the file to my local computer. Then it will play OK. Works more often than not. <hr /></blockquote>
That was the first thing I tried. Unfortnately, that didn't work either (for any of the players).

Thanks,
Dave

Bob_Jewett
08-12-2005, 09:40 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr>... We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail.
... <hr /></blockquote>
Actually, this is not entirely true. The game that is most interested in the exact path of the cue ball after contact with an object ball is English Billiards, and the writers on that subject uniformly recommend inside english (they call it something else, of course) to close the carom angle of the cue ball for a half-ball contact, and outside english to open that angle.

There are two things at work for this effect. The first is the swerve of the cue ball on the way to the object ball. This directly changes the incoming angle on the cue ball and equally changes the outbound path.

The second effect is due to throw. Just as inside english causes the object ball to be dragged towards the cue ball's outbound path, it also pulls the cue ball's path towards the departing object ball. If the shot is a stun shot, the cue ball departs at 90 degrees to the line of centers of the balls at impact (which is not 90 degrees from the object ball's path), so the direction is not changed -- it is just slowed down some. If the cue ball has follow or draw, there will be less sideways speed and the departing angle will be narrowed a little. This is a very simple consequence of conservation of momentum.

Similarly, outside english speeds up the cue ball's sideways motion and generally widens its departing angle.

The effect is not a large one, but on a 12-foot table it is large enough to cause misses or makes.

And I think it's important to repeat: you almost never, ever play with a level stroke. That's why english shots swerve. If you have negative elevation -- think about how you can get this -- you can make the cue ball curve to the left with right english.

Qtec
08-12-2005, 09:55 AM
Dave, just click and wait.

Qtec

dr_dave
08-12-2005, 10:02 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>... We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail.<hr /></blockquote>
Actually, this is not entirely true. The game that is most interested in the exact path of the cue ball after contact with an object ball is English Billiards, and the writers on that subject uniformly recommend inside english (they call it something else, of course) to close the carom angle of the cue ball for a half-ball contact, and outside english to open that angle.
...
Just as inside english causes the object ball to be dragged towards the cue ball's outbound path, it also pulls the cue ball's path towards the departing object ball. If the shot is a stun shot, the cue ball departs at 90 degrees to the line of centers of the balls at impact (which is not 90 degrees from the object ball's path), so the direction is not changed -- it is just slowed down some. If the cue ball has follow or draw, there will be less sideways speed and the departing angle will be narrowed a little. This is a very simple consequence of conservation of momentum.

Similarly, outside english speeds up the cue ball's sideways motion and generally widens its departing angle.<hr /></blockquote>
FYI, for those that are interested, I have illustrations and explanations for these effects posted online in my May, 2005 instructional article (http://www.engr.colostate.edu/~dga/pool/bd_articles/may05.pdf).
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>If you have negative elevation -- think about how you can get this -- you can make the cue ball curve to the left with right english.<hr /></blockquote>
Use a short stick so it doesn't need to be elevated above the rail (i.e., have the butt end touching or close to the table surface). Do I get a gold star for my answer?

Regards,
Dave

Fred Agnir
08-12-2005, 10:07 AM
<blockquote><font class="small">Quote dr_dave:</font><hr>
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>If you have negative elevation -- think about how you can get this -- you can make the cue ball curve to the left with right english.<hr /></blockquote>
Use a short stick so it doesn't need to be elevated above the rail (i.e., have the butt end touching or close to the table surface). Do I get a gold star for my answer?

Regards,
Dave <hr /></blockquote>The easiest way for demonstration purposes if to put the cueball on a piece of chalk on the rail. Crouch down and shoot upwards at the cueball. Right-hand english from this angle will make the cueball swerve to the left. This exercise also shows the dramatic effect of squirt.

Fred

dr_dave
08-12-2005, 10:09 AM
<blockquote><font class="small">Quote Qtec:</font><hr> Dave, just click and wait.<hr /></blockquote>
I tried that too. I would think you would have more faith in me than that.
<blockquote><font class="small">Quote Qtec:</font><hr>No luck? Download the VLC media player.[ it plays just about anything] Go here. (http://www.videolan.org/)<hr /></blockquote>
Thanks for the info, but I refuse to do this in protest of the posters of the video. I believe people should post only formats that are playable by players that people already have on their computers (e.g., Windows Media Player or Quicktime). I already wasted about 30 minutes trying to get the video to play. IMHO, that is unacceptable in today's world.

Regards,
Dave

Fred Agnir
08-12-2005, 10:22 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> I agree with Fred that we don't want this thread to become an argument over what "precession" means. If anbody really wants to know the accepted definition, they can refer to Webster's dictionary.
<hr /></blockquote>I said this before about "Custom vs. Production," and I'll say it again. The two little old ladies that do the Webster's Dictionary definitions don't normally get things right when certain words are used in certain industries. That is, they give the most general meaning, and can't be held as understanding industry-specific definitions. That being said, all rotating object can undergo precession. This is very well accepted in the gyroscope studies and the gyrsocope world, a study that includes planes and rotating planets, and not just tops with fixed ends.

Here's yet another site that explains precession in its first couple of sentence, again, 100% in line with Billy Bob's quotation.

http://encyclopedia.laborlawtalk.com/Precession

Fred

Chopstick
08-12-2005, 10:24 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Qtec:</font><hr> Dave, just click and wait.<hr /></blockquote>
I tried that too. I would think you would have more faith in me than that.
<blockquote><font class="small">Quote Qtec:</font><hr>No luck? Download the VLC media player.[ it plays just about anything] Go here. (http://www.videolan.org/)<hr /></blockquote>
Thanks for the info, but I refuse to do this in protest of the posters of the video. I believe people should post only formats that are playable by players that people already have on their computers (e.g., Windows Media Player or Quicktime). I already wasted about 30 minutes trying to get the video to play. IMHO, that is unacceptable in today's world.

Regards,
Dave <hr /></blockquote>

Come on Dave. I played it with Windows media player. You just need to update your software. If there's a better player out there, it makes sense to use it. I downloaded it.

dr_dave
08-12-2005, 10:27 AM
Fred,

Thanks for the link. I might use it in my course this semester. The site does an excellent job of describing and illustrating all of the effects. The abundance of links to additional info is awesome.

Thanks,
Dave

<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> I agree with Fred that we don't want this thread to become an argument over what "precession" means. If anbody really wants to know the accepted definition, they can refer to Webster's dictionary.
<hr /></blockquote>I said this before about "Custom vs. Production," and I'll say it again. The two little old ladies that do the Webster's Dictionary definitions don't normally get things right when certain words are used in certain industries. That is, they give the most general meaning, and can't be held as understanding industry-specific definitions. That being said, all rotating object can undergo precession. This is very well accepted in the gyroscope studies and the gyrsocope world, a study that includes planes and rotating planets, and not just tops with fixed ends.

Here's yet another site that explains precession in its first couple of sentence, again, 100% in line with Billy Bob's quotation.

http://encyclopedia.laborlawtalk.com/Precession

Fred <hr /></blockquote>

Fred Agnir
08-12-2005, 10:37 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> Thanks for the info, but I refuse to do this in protest of the posters of the video. I believe people should post only formats that are playable by players that people already have on their computers (e.g., Windows Media Player or Quicktime). I already wasted about 30 minutes trying to get the video to play. IMHO, that is unacceptable in today's world.

Regards,
Dave <hr /></blockquote> Unfortunately, with the onslaught of various recording devices, the formats change rapidly. This is an AVI file, a format which can get updates every week. Usuallly, all you need to do is update your Codecs, whether you're using Windows Media Player or RealPlayer. Updating Codecs is something everyone should be doing regulary, just because of these incidents.

Fred

Qtec
08-12-2005, 10:48 AM
[ QUOTE ]
Come on Dave. I played it with Windows media player. You just need to update your software. If there's a better player out there, it makes sense to use it. I downloaded it.

<hr /></blockquote>

It works for me too. The VLC is free with no spyware. it plays 99.9% of all video formats. Its the best, most consistant player on the web.

Qtec

Fran Crimi
08-12-2005, 10:56 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used.

Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail. <font color="blue"> Quick question: Are you referring here to the use of a level stroke or an elevated stroke as well?

Fran</font color>

Fred <hr /></blockquote> <hr /></blockquote>I was talking about "level," but I guess I needed to make sure I was talking about the cueball "curving" due to english.<hr /></blockquote>
I usually assume that when somebody says "level stroke" they mean "minimal cue stick elevation" or "keep the cue stick as level as possible at impact with the cue ball," but I can see why there is confusion. By "level stroke," do you and Fran mean "drop your elbow during the stroke so the cue stick remains at the same angle during the stroke?" Please clarify.

Thanks,
Dave <hr /></blockquote>

The way I understand 'level' is the cue stick is perfectly level (or as perfectly level as a human can get it) throughout the period of time the cue tip is in contact with the cue ball.

Elbow dropping is one way to accomplish this; also through back hand manipulation, if the stroke is short enough.

Fran

dr_dave
08-12-2005, 01:01 PM
Fran,

Thanks for clarifying. It sounds like you might be referring to the levelness of the cue stick just during impact with the cue ball (which only lasts about a thousandth of a second). In that case, what the stroke does before and after impact is really not in question. From now on, I will be more careful when describing cue stick elevation and when using the term "level," especially when the discussion might involve pendulum vs. piston stroking.

Thanks,
Dave

<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used.

Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail. <font color="blue"> Quick question: Are you referring here to the use of a level stroke or an elevated stroke as well?

Fran</font color>

Fred <hr /></blockquote> <hr /></blockquote>I was talking about "level," but I guess I needed to make sure I was talking about the cueball "curving" due to english.<hr /></blockquote>
I usually assume that when somebody says "level stroke" they mean "minimal cue stick elevation" or "keep the cue stick as level as possible at impact with the cue ball," but I can see why there is confusion. By "level stroke," do you and Fran mean "drop your elbow during the stroke so the cue stick remains at the same angle during the stroke?" Please clarify.

Thanks,
Dave <hr /></blockquote>

The way I understand 'level' is the cue stick is perfectly level (or as perfectly level as a human can get it) throughout the period of time the cue tip is in contact with the cue ball.

Elbow dropping is one way to accomplish this; also through back hand manipulation, if the stroke is short enough.

Fran <hr /></blockquote>

Fran Crimi
08-12-2005, 01:28 PM
<blockquote><font class="small">Quote dr_dave:</font><hr> Fran,

Thanks for clarifying. It sounds like you might be referring to the levelness of the cue stick just during impact with the cue ball (which only lasts about a thousandth of a second). In that case, what the stroke does before and after impact is really not in question. From now on, I will be more careful when describing cue stick elevation and when using the term "level," especially when the discussion might involve pendulum vs. piston stroking.

Thanks,
Dave

<hr /></blockquote>

Well, it's not that simple. There is a process that has to take place in order to accomplish a level cue exactly at impact. You don't just shoot with a piston-type stroke and then suddenly level off at impact. The process that leads to obtaining level at impact can't be ignored, with the follow-through position being a further gauge indicating that the process took place.

Fran

dr_dave
08-12-2005, 01:52 PM
<blockquote><font class="small">Quote Qtec:</font><hr><blockquote><font class="small">Quote Chopstick:</font><hr>Come on Dave. I played it with Windows media player. You just need to update your software. If there's a better player out there, it makes sense to use it. I downloaded it.<hr /></blockquote>

It works for me too. The VLC is free with no spyware. it plays 99.9% of all video formats. Its the best, most consistant player on the web.<hr /></blockquote>
Ok. You guys finally wore me down. I downloaded the VLC player and it worked with the "precession" video clip.

However, I still would not describe what I see as "precession." What I see is a combination of topspin and sidespin, where the topspin wears off faster than the sidespin. Each time the cue ball hits the rail, it loses a little of both spins. Between rail contacts, the friction between the ball and the cloth (due to sliding contact) creates a torque that slows the topspin. The torque acts about the same axis as the topspin; so there are no gyroscopic effects ... the torque just slows the topspin. There is also a small friction torque that slows the sidespin; but again, I still don't think there are any gyroscopic effects because the friction torques always act in the same directions (because the point of contact is always directly below the center of the ball, and the spin directions don't change ... only the spin speeds change ... they decrease, albeit at different rates).

Now, the stripe around the Elephant Practice Ball creates an interesting visual effect that makes it look like the spin axis is wobbling, but I don't think it is. The spin axis is just slowly changing direction towards vertical as the topspin wears off faster than the sidespin. I think if a red-dot Aramith cue ball were used instead, the axis of rotation might be easier to see. I plan to use the red-dot ball for all future filming, because I think it gives better visual feedback than the black stripe (regardless of how the ball is oriented relative to the spin axis). A good example where the black strip doesn't always give good feedback is when the black stripe happens to be in the spin direction (e.g., see NV 3.7 (http://www.engr.colostate.edu/~dga/pool/normal_videos/NV3-7.htm))

FYI, I have a high-speed video clip of a similar shot, with just topspin. It can be found at HSV 4.3 (http://www.engr.colostate.edu/~dga/pool/high_speed_videos/HSV4-3.htm).

Regards,
Dave

dr_dave
08-12-2005, 01:55 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> Fran,

Thanks for clarifying. It sounds like you might be referring to the levelness of the cue stick just during impact with the cue ball (which only lasts about a thousandth of a second). In that case, what the stroke does before and after impact is really not in question. From now on, I will be more careful when describing cue stick elevation and when using the term "level," especially when the discussion might involve pendulum vs. piston stroking.

Thanks,
Dave

<hr /></blockquote>

Well, it's not that simple. There is a process that has to take place in order to accomplish a level cue exactly at impact. You don't just shoot with a piston-type stroke and then suddenly level off at impact. The process that leads to obtaining level at impact can't be ignored, with the follow-through position being a further gauge indicating that the process took place.<hr /></blockquote>
Agreed. That's all the cue ball cares about is tip placement, speed, and direction at impact. But to create the desired placement, speed, and direction, stroke mechanics can be very important, especially for consistency and accuracy.

Respectfully,
Dave

Bob_Jewett
08-12-2005, 02:12 PM
<blockquote><font class="small">Quote Fran Crimi:</font><hr> ...
The way I understand 'level' is the cue stick is perfectly level (or as perfectly level as a human can get it) throughout the period of time the cue tip is in contact with the cue ball.
... <hr /></blockquote>
That's the way I understand it as well.

A level stroke almost never occurs because the rail is in the way. Even when the rail is not in the way, your knuckles are in the way. When the stick is level, the distance from the center of the tip to the bed cloth is the same as the distance of the center of the screw that holds the bumper on the butt to the bed cloth (or the extended plane of the cloth). I hope this last statement is obvious once it is stated.

Once again: 99.999% of all strokes are elevated, and the remainder are trick demonstrations.

If you don't believe me -- and I'm sure many don't -- get into a position you think is level and have someone measure the two distances. The difference between the two distances in inches gives the elevation in degrees, assuming a 57-inch cue stick. You really have to work to get down to two degrees of elevation for draw shots, and a level stroke is nearly impossible, even in a trick demonstration with a funny grip.

Bob_Jewett
08-12-2005, 02:34 PM
<blockquote><font class="small">Quote Billy_Bob:</font><hr> From...
Mechanics of Billiards, and Analysis of Willie Hoppe's Stroke - Moore, A. D. - 1942.

"The Ball as a Gyroscope"
"When the billiard ball spins, it is a gyroscope: its spin axis tends to remain fixed on the same distant point in space. Next if a force is applied that tends to shift the axis, two things happen: the force is resisted, and there is precession of the axis. Precession means that instead of the axis shifting the way the force tries to shift it, it shifts instead at right angles to the force tendency and continues to do so as long as the shift force is applied."

Note: The above is a paper by... A. D. Moore, Professor of Electrical Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan - January 17, 1942.

The copy I have seems to be a copy of a copy of a copy of a typewritten paper. It is almost unreadable.

If someone could get a hold of a clean copy of the original from the University of Michigan Engineering Library, then post it on the internet, I think it would be a good thing (as Martha Stewart would say...). Then maybe I could read it!

(I bought mine from someone on Ebay.)
<hr /></blockquote>
Moore got several things wrong in his paper, and I think this "precession" thing was one of them. As has been pointed out by others, precession is normally a wobble of the axis in a circle around the force vector that's acting on a top-like object. Coriolis predicts that the axis of spin of a ball coming to equilibrium (with rolling follow and some side spin) will move in a great circle from its starting position to its equilibrium position, and then it will remain more or less constant as the spin slowly wears off*. That is not precession as I understand the term.

I have a copy of Moore's paper that I believe came directly from Moore (to Clem Trainer, originally), and it is almost as illegible as the copies in circulation.

(* Actually, during the slow rolling to a stop phase, the axis usually shifts gradually to one of two positions: polar if there is more spin than speed, or horizontal and perpendicular to the path if there is more speed than spin.)

SpiderMan
08-12-2005, 02:46 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> think about how you can get this -- you can make the cue ball curve to the left with right english. <hr /></blockquote>

Just hit upwards on the cueball. One way would be to use a very short stick, as Dave noted. Another would be to remove a rail for the shot, or perhaps place the cueball on top of the rail /ccboard/images/graemlins/grin.gif

SpiderMan

SpiderMan
08-12-2005, 02:53 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> ...
The way I understand 'level' is the cue stick is perfectly level (or as perfectly level as a human can get it) throughout the period of time the cue tip is in contact with the cue ball.
... <hr /></blockquote>
That's the way I understand it as well.

A level stroke almost never occurs because the rail is in the way. Even when the rail is not in the way, your knuckles are in the way. When the stick is level, the distance from the center of the tip to the bed cloth is the same as the distance of the center of the screw that holds the bumper on the butt to the bed cloth (or the extended plane of the cloth). I hope this last statement is obvious once it is stated.

Once again: 99.999% of all strokes are elevated, and the remainder are trick demonstrations.

If you don't believe me -- and I'm sure many don't -- get into a position you think is level and have someone measure the two distances. The difference between the two distances in inches gives the elevation in degrees, assuming a 57-inch cue stick. You really have to work to get down to two degrees of elevation for draw shots, and a level stroke is nearly impossible, even in a trick demonstration with a funny grip. <hr /></blockquote>

I don't think the stick must be level in order to meet the "level stroke" requirement, at least in theory, from the point of view of the cueball.

The cueball would call a "level stroke" anything that had the tip's contact point moving parallel to the cloth at the moment of contact. It doesn't know about the attitude of the rest of the cue. This "level stroke" could occur despite the butt of the cue being elevated.

One way for this to happen would be a coordinated dropping of the butt during the stroke. Difficult to achieve, and I'm not arguing for it, but it probably happens for some players who utilize an elbow drop.

SpiderMan

Bob_Jewett
08-12-2005, 04:01 PM
<blockquote><font class="small">Quote SpiderMan:</font><hr>...
The cueball would call a "level stroke" anything that had the tip's contact point moving parallel to the cloth at the moment of contact. It doesn't know about the attitude of the rest of the cue. This "level stroke" could occur despite the butt of the cue being elevated.

One way for this to happen would be a coordinated dropping of the butt during the stroke. Difficult to achieve, and I'm not arguing for it, but it probably happens for some players who utilize an elbow drop. <hr /></blockquote>
So the cue stick would be pivoting about the bridge hand during the shot or the bridge hand could be rising as the stick comes forward. Maybe those players whose bridge hands end a foot in the air are onto something.

While this seems to be a reasonable idea, it's not clear to me that it's correct. The experiment is hard to do. My objection is that the cue stick can push the ball forward far more efficiently than sideways, and the rotation part of the shot is effectively pushing the ball sideways.

dr_dave
08-13-2005, 06:45 AM
Bob,

Well stated. I agree with you per my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1).

Regards,
Dave

Dave
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Billy_Bob:</font><hr> From...
Mechanics of Billiards, and Analysis of Willie Hoppe's Stroke - Moore, A. D. - 1942.

"The Ball as a Gyroscope"
"When the billiard ball spins, it is a gyroscope: its spin axis tends to remain fixed on the same distant point in space. Next if a force is applied that tends to shift the axis, two things happen: the force is resisted, and there is precession of the axis. Precession means that instead of the axis shifting the way the force tries to shift it, it shifts instead at right angles to the force tendency and continues to do so as long as the shift force is applied."

Note: The above is a paper by... A. D. Moore, Professor of Electrical Engineering, College of Engineering, University of Michigan, Ann Arbor, Michigan - January 17, 1942.

The copy I have seems to be a copy of a copy of a copy of a typewritten paper. It is almost unreadable.

If someone could get a hold of a clean copy of the original from the University of Michigan Engineering Library, then post it on the internet, I think it would be a good thing (as Martha Stewart would say...). Then maybe I could read it!

(I bought mine from someone on Ebay.)
<hr /></blockquote>
Moore got several things wrong in his paper, and I think this "precession" thing was one of them. As has been pointed out by others, precession is normally a wobble of the axis in a circle around the force vector that's acting on a top-like object. Coriolis predicts that the axis of spin of a ball coming to equilibrium (with rolling follow and some side spin) will move in a great circle from its starting position to its equilibrium position, and then it will remain more or less constant as the spin slowly wears off*. That is not precession as I understand the term.

I have a copy of Moore's paper that I believe came directly from Moore (to Clem Trainer, originally), and it is almost as illegible as the copies in circulation.

(* Actually, during the slow rolling to a stop phase, the axis usually shifts gradually to one of two positions: polar if there is more spin than speed, or horizontal and perpendicular to the path if there is more speed than spin.) <hr /></blockquote>

dr_dave
08-13-2005, 06:50 AM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>Once again: 99.999% of all strokes are elevated, and the remainder are trick demonstrations.

If you don't believe me -- and I'm sure many don't -- get into a position you think is level and have someone measure the two distances. The difference between the two distances in inches gives the elevation in degrees, assuming a 57-inch cue stick. You really have to work to get down to two degrees of elevation for draw shots, and a level stroke is nearly impossible, even in a trick demonstration with a funny grip.<hr /></blockquote>
If people don't believe it, they can see an illustration and math to prove it at TP A.3 (http://www.engr.colostate.edu/~dga/pool/technical_proofs/new/TP_A-3.pdf).

Dave

Jal
08-13-2005, 05:22 PM
<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote Jal:</font><hr> I still think that sidespin will not affect the path after collision with an object ball. The torque produced by the friction will lay in the vertical plane in which the friction vector is embedded. <hr /></blockquote>I think you're dead on with this. I was thinking a little more on this, and any axis tilt would be in a plane along the tangent, so the path would be unaltered. How about off a cushion?

Fred <hr /></blockquote>

There's a strange sort of spin component that's added to a cueball that has follow or draw when it collides with a cushion, I think. (The same with an object ball that has follow.) It happens when it comes in at some angle other than the perpendicular.

The upward or downward facing frictional force between the cueball and cushion produces a torque which is oriented parallel to the rail. This has a component parallel to the direction of the cueball, in both the in-bound and out-bound phases (if it still retains some draw of follow). If you would look at the cueball from behind as from a player's normal perspective, and if you could filter out the other spin components, you would see the cueball begin to rotate clockwise or counter clockwise. It would depend on whether the ball had follow or draw and if it was approaching the cushion from left-to-right or right-to-left.
It's effect should be that the original spin axis will swing around to point more in the direction (or anti-direction) the cueball is moving.

At least this is what I think and I viewed several of Dr. Dave's videos to see if this effect could be discerned. It's pretty hard to tell for the most part because the induced sidespin also "pulls" the axis in roughly the same direction (and upward). But I think this is a rather bizarre example of it:

http://www.engr.colostate.edu/~dga/pool/high_speed_videos/new/HSVA-32.htm

As you can see it appears that the cueball comes off the rail with some backspin relative to its new direction. It seems to be a pretty rare case of it being struck above center (by the cushion) with the result being draw! If the above explanation is correct, the torque, which in this case is pointing to the right along the cushion, has swung the north pole of the original spin axis, which was pointing to the left originally, most of the way around to the right. What we see after the impact is the south pole pointing to the left and somewhat upward from the induced sidespin.

Maybe you or others disagree with this interpretation?

Jim

Stretch
08-13-2005, 05:27 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Fran Crimi:</font><hr> ...
The way I understand 'level' is the cue stick is perfectly level (or as perfectly level as a human can get it) throughout the period of time the cue tip is in contact with the cue ball.
... <hr /></blockquote>
That's the way I understand it as well.

A level stroke almost never occurs because the rail is in the way. Even when the rail is not in the way, your knuckles are in the way. When the stick is level, the distance from the center of the tip to the bed cloth is the same as the distance of the center of the screw that holds the bumper on the butt to the bed cloth (or the extended plane of the cloth). I hope this last statement is obvious once it is stated.

Once again: 99.999% of all strokes are elevated, and the remainder are trick demonstrations.

If you don't believe me -- and I'm sure many don't -- get into a position you think is level and have someone measure the two distances. The difference between the two distances in inches gives the elevation in degrees, assuming a 57-inch cue stick. You really have to work to get down to two degrees of elevation for draw shots, and a level stroke is nearly impossible, even in a trick demonstration with a funny grip. <hr /></blockquote>

So the best you can hope for given the rails at best is what, 5 deg. of elevation? Sounds about right i guess. But i was wondering. You know how a scoop shot works, your hitting it so low that the cb is forced up, so there is a certain amount of lift with a lower than it's centre of gravity hit. Anyway, is there a point where the downward angle of attack is cancelled out by the lift factor? Sometimes i think there is and i call it the sweete spot. You can hear it by the sound your tip makes at impact. When i get that great crisp sound i know i'm in stroke. St.

Fran Crimi
08-13-2005, 07:23 PM
I don't think people have a probelem in knowing that when the rail is in play, then you can't have a level stroke, particularly when you're shooting at center ball. I don't think you'll have any non-believers there.

My reference to 'level' is with shots where the rail is not in play and the shooter is shooting with topspin. There's no reason why you can't shoot with a level stroke shooting at the top of the cue ball with no obstructions. There's plenty of room for your back hand to fit between your cue and the table. And if it doesn't meet the standard of being perfectly level, it's good enough for me to call 'level.' I'm not about to change the word to 'almost perfectly level.'

Fran

heater451
08-14-2005, 12:01 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>Moore got several things wrong in his paper, and I think this "precession" thing was one of them. As has been pointed out by others, precession is normally a wobble of the axis in a circle around the force vector that's acting on a top-like object. Coriolis predicts that the axis of spin of a ball coming to equilibrium (with rolling follow and some side spin) will move in a great circle from its starting position to its equilibrium position, and then it will remain more or less constant as the spin slowly wears off*. That is not precession as I understand the term. . . .(* Actually, during the slow rolling to a stop phase, the axis usually shifts gradually to one of two positions: polar if there is more spin than speed, or horizontal and perpendicular to the path if there is more speed than spin.) <hr /></blockquote>Is the difference here relative to the "base" of the axis?---That is, if the end of the axis-of-spin at the top of the ball moves, while the end of the axis that meets the table (basically) stays fix, it's "precession", but if the ends of the axis-of-spin move around the center of the sphere equally (opposite distances from the center, but with the same rotational directon), then it's "coriolis effect"?

================================

Bob_Jewett
08-14-2005, 09:01 PM
<blockquote><font class="small">Quote heater451:</font><hr> ... Is the difference here relative to the "base" of the axis?---That is, if the end of the axis-of-spin at the top of the ball moves, while the end of the axis that meets the table (basically) stays fix, it's "precession", but if the ends of the axis-of-spin move around the center of the sphere equally (opposite distances from the center, but with the same rotational directon), then it's "coriolis effect"? <hr /></blockquote>
I'm not sure what you're asking. The ball has only one axis of rotation at any instant.

And to the best of my knowledge, the Coriolis effect relates to the independent movement of something on a rotating object (such as wind on the Earth) and has nothing to with the axis of rotation of a cue ball. The Coriolis effect will cause a ball to curve as it crosses a table, because the Earth is rotating under the ball, but this effect is not large enough to notice. Notice that the direction of the curve depends on whether you are shooting north or south.

heater451
08-15-2005, 06:54 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote heater451:</font><hr> ... Is the difference here relative to the "base" of the axis?---That is, if the end of the axis-of-spin at the top of the ball moves, while the end of the axis that meets the table (basically) stays fix, it's "precession", but if the ends of the axis-of-spin move around the center of the sphere equally (opposite distances from the center, but with the same rotational directon), then it's "coriolis effect"? <hr /></blockquote>
I'm not sure what you're asking. The ball has only one axis of rotation at any instant.

And to the best of my knowledge, the Coriolis effect relates to the independent movement of something on a rotating object (such as wind on the Earth) and has nothing to with the axis of rotation of a cue ball. The Coriolis effect will cause a ball to curve as it crosses a table, because the Earth is rotating under the ball, but this effect is not large enough to notice. Notice that the direction of the curve depends on whether you are shooting north or south. <hr /></blockquote>Ah--I had looked up the Coriolis effect a few days ago into this thread, and didn't think that it related like Fred thought--for the reason you state here. The example I read had to do with throwing a ball from a moving carousel.

In seeking to re-phrase my question, I wound up checking the Wikipedia about precession (http://en.wikipedia.org/wiki/Precession), and somewhat answered my own question. Unfortunately, it lead me back to something that I was trying to figure out mathmatically (with the help of the TI-89 Titanium graphing calculator!), and I'm still not sure enough about how to phrase my thoughts. (I need to have my brother create a 3-D model for me. . . .)

Last year some time, Fred started a thread about sidespin and it's effect on swerve last year some time, and we got to precession, but I didn't have time to keep up with the thread. And, you've already answered that question in your post about English Billiards. (It escaped me last year, but I thought distance would be a factor, which is supported by you: "The effect is not a large one, but on a 12-foot table it is large enough to cause misses or makes.") In any case, I think that once the ball begins forward movement, then any "level" sidespin immediately becomes "up-spin", such as you would get with your idea of shooting with "negative elevation", and thus the ball path would be affected, however small it is.

Of course, with a constant speed/spin ratio, I don't think the path would be affected, since then the gyroscopic precession would counter-effect itself as the ball traveled. However, as one or both spins (side and forward) decay, the path becomes 'submissive' to whichever force is stronger at the time--which I think someone has already said somewhere in this thread.

To me, it seems that if you strike the cb with (level) left english, as the ball moves forward, then the movement of the strike point would describe a sine wave. My thinking being that you could combine two waves to find it: The first wave would be described by the movement of a point on the circumference of the ball, in the vertical plane which bisects the sphere--appearing as a vertical line in the rear view as the ball rolls forward (is there a term for the "vertical equator"?) . The second the wave would be described by the movement of a point on the equator of the ball, as it spins on a vertical axis. If we start with the same point in each graph, say the center-ball strike point, and retain the 1:1 S/S ratio, then I think the graphs would share the same period and frequency, but be at 90-deg angles to one another, with the graphs crossing at every rotation of the ball--about every 7.065", for a 2.25" dia ball. So, the actual graph would be the same period and frequency, but it would be 45-deg between the X and Y planes.

I will stop here, because I don't know if my understanding of combining the waves in 3-D is correct. . . .

Let me know, if this makes ANY sense.

============================

Fred Agnir
08-16-2005, 06:48 AM
<blockquote><font class="small">Quote heater451:</font><hr> Ah--I had looked up the Coriolis effect a few days ago into this thread, and didn't think that it related like Fred thought--for the reason you state here. <hr /></blockquote> I see this thread has gone by the wayside. I didn't mean "Coriolis Effect." I mean that the motion of the rotating cueball when it hits another object ball reacts just as Coriolis describes it.

Fred Agnir
08-16-2005, 06:51 AM
<blockquote><font class="small">Quote heater451:</font><hr> Ah--I had looked up the Coriolis effect a few days ago into this thread, and didn't think that it related like Fred thought--for the reason you state here. <hr /></blockquote> I see this thread has gone by the wayside. I didn't mean "Coriolis Effect." I mean that the motion of the rotating cueball when it hits another object ball reacts just as Coriolis describes it. The axis will tilt, as it should tilt, as described by Coriolis, and what is described as "gyroscopic precession for a non-fixed axis rotating body." People who don't have an understanding on this should do a study on planes or cars and how they travel and tilt, when subject to forces. Planes and cars don't spiral with a decaying axis. They just tilt when forces hits them. When the force stops, they stop tilting. Easy as that. The tilting has always been called "Gyroscopic Precession."

To be clear, the precession on the cueball only happens when the contact happens. After contact, the axis NO LONGER CONTINUES TO TILT.

[ QUOTE ]
In seeking to re-phrase my question, I wound up checking the Wikipedia about precession (http://en.wikipedia.org/wiki/Precession), and somewhat answered my own question. <hr /></blockquote> This definition again is 100% consistent with what I've said all along.

[ QUOTE ]
Last year some time, Fred started a thread about sidespin and it's effect on swerve last year some time,<hr /></blockquote> There is no effect if the cue is level, which of course is hard to do. But if the cue was level, there would be no swerve due to the english. Even with, say, low-left or high-right.

[ QUOTE ]
and we got to precession, <hr /></blockquote> As far as I can tell, the cueball axis does not precess without interaction with any other forces.

Fred

Fred Agnir
08-16-2005, 06:56 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> Bob,

Well stated. I agree with you per my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1).<blockquote><font class="small">Quote Bob_Jewett:</font><hr> Moore got several things wrong in his paper, and I think this "precession" thing was one of them. As has been pointed out by others, precession is normally a wobble of the axis in a circle around the force vector that's acting on a top-like object. <hr /></blockquote><hr /></blockquote>This is wrong. As I have pointed out many times, precession has never been exclusively coupled to "top-like" objects. This is where you and Bob are incorrect and confusing the issue. Check Wikepedia. Check plane travel. So, this being said, I think you both need to recheck the meaning of precession and adjust your responses accordingly.

Fred

dr_dave
08-16-2005, 08:11 AM
Fred,

I agree with you that gyroscopic effects and precession can apply to non-top-like objects. Gyroscopic effects can also apply to any rotating equipment on frames that can rotate in other directions (e.g., gyroscopes or any rotating parts on aircraft, boats, and vehicles). Gyroscopic effects also come into play in bowling, because bowling balls, unlike pool balls, are not homogeneous spheres. High-end bowling balls have asymmetric cores inside that help create a slight wobble in the ball's spin axis to help improve traction by preventing the ball from tracking in one circle (where oil from the lane can build up and reduce friction). However, I still stand by my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1) concerning the motion of pool balls. If you disagree with anything in my message (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1), please let me know.

Regards,
Dave

<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> Bob,

Well stated. I agree with you per my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1).<blockquote><font class="small">Quote Bob_Jewett:</font><hr> Moore got several things wrong in his paper, and I think this "precession" thing was one of them. As has been pointed out by others, precession is normally a wobble of the axis in a circle around the force vector that's acting on a top-like object. <hr /></blockquote><hr /></blockquote>This is wrong. As I have pointed out many times, precession has never been exclusively coupled to "top-like" objects. This is where you and Bob are incorrect and confusing the issue. Check Wikepedia. Check plane travel. So, this being said, I think you both need to recheck the meaning of precession and adjust your responses accordingly.

Fred <hr /></blockquote>

Fred Agnir
08-16-2005, 08:31 AM
<blockquote><font class="small">Quote dr_dave:</font><hr> However, I still stand by my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1) concerning the motion of pool balls. <hr /></blockquote> I agree that the total motion shown in that video is not precession. However, the initial axis tilt upon contact with the object ball is. And it's the initial tilt and only that initial tilt that I brought up in the first place.

Fred

dr_dave
08-16-2005, 08:46 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr> <blockquote><font class="small">Quote dr_dave:</font><hr> However, I still stand by my other posting (http://www.billiardsdigest.com/ccboard/showthreaded.php?Cat=&amp;Board=ccb&amp;Number=202956&amp;page =0&amp;view=collapsed&amp;sb=5&amp;o=&amp;vc=1) concerning the motion of pool balls. <hr /></blockquote> I agree that the total motion shown in that video is not precession. However, the initial axis tilt upon contact with the object ball is. And it's the initial tilt and only that initial tilt that I brought up in the first place.<hr /></blockquote>
Fred,

Thank you for the clarification. I am sorry if I misinterpreted anything in previous messages. I agree with you that the cue ball spin axis changes direction as a result of impact with the object ball (due to frictional impulse forces), but I still would not call that "precession." But it sounds like we will need to agree to disagree on this one. To me, "precession" implies a gradual, and often continual, motion of a spin axis. With impact, the frictional force creates an almost instantaneous change in angular momentum, which corresponds to an almost instantaneous change in the spin axis direction. There are no gyroscopic effects at play. I say this, because in my experience, "precession" is often described as a gyroscopic effect.

Respectfully,
Dave

PS: Fred, thank you for helping to keep us all sharp and open-minded with your questioning. Sometimes, things seem to get a little over-heated, but debate and discussion is always valuable. Thanks again, Dave.

Bob_Jewett
08-16-2005, 11:51 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr> ... As I have pointed out many times, precession has never been exclusively coupled to "top-like" objects. <hr /></blockquote>

Well, yes, but as soon as someone says "precession" most listeners expect some form of bizarre behaviour like the bicycle wheel hidden in the suitcase or a top wobbling about its obvious major axis.

<blockquote><font class="small">Quote Fred Agnir:</font><hr> This is where you and Bob are incorrect and confusing the issue.<hr /></blockquote>
I think anyone who introduces the word precession into a discussion of spin on a billiard ball is confusing the issue. Precession as such is not useful in understanding billiard physics, in my opinion. It is much easier to separate out the spins on the three axes and treat them more or less separately.

A main thing to remember is that a uniform sphere, such as a pool ball, has nothing identifiable as a major axis.

Fred Agnir
08-16-2005, 12:49 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr>

<blockquote><font class="small">Quote Fred Agnir:</font><hr> This is where you and Bob are incorrect and confusing the issue.<hr /></blockquote>
I think anyone who introduces the word precession into a discussion of spin on a billiard ball is confusing the issue. <hr /></blockquote> Oh, I 100% agree with you. In fact I already said that same thing in this thread. The use of the word "precession" is confusing. That's why I didn't want to bring up the word.

Unfortunately, someone brought it up, and the initial question got lost in the melée. Fortunately, I think I got my question answered, but still wouldn't mind seeing some more high speed video.

Fred &lt;~~~ thought I posted a response already

wolfdancer
08-16-2005, 01:32 PM
I thought precession was the time period, shortly after you elect a Republican President, and the inevitable recession that follows???

wolfdancer
08-16-2005, 01:42 PM
Just my opinion.....but I think this thread has been as enlightening to the avg. pool player, as a treatise on ballistics would have been to Willie Mays.....or any dog catching a Frisbee. We could throw in some aerodynamics for the dog.

heater451
08-16-2005, 04:40 PM
<blockquote><font class="small">Quote Fred Agnir:</font><hr> I see this thread has gone by the wayside. I didn't mean "Coriolis Effect." I mean that the motion of the rotating cueball when it hits another object ball reacts just as Coriolis describes it.<hr /></blockquote>"The wayside" doesn't matter, in online forums. /ccboard/images/graemlins/smile.gif Besides, I think that clearing up misconceptions along the way in a thread, still contributes to the whole of the thinking--messy, but positive.

Anyway, I by re-reading your quote of me, I see that I wrote it badly. I wasn't saying that I disagreed with you about the Coriolis effect or "affect" (certainly not"Effect"). I was trying to say that I understood it the same as you.
<blockquote><font class="small">Quote Fred:</font><hr>The axis will tilt, as it should tilt, as described by Coriolis, and what is described as "gyroscopic precession for a non-fixed axis rotating body."<hr /></blockquote>Not sure if you mean "Coriolis", the person here. . . .

<blockquote><font class="small">Quote Fred:</font><hr>To be clear, the precession on the cueball only happens when the contact happens. After contact, the axis NO LONGER CONTINUES TO TILT.<hr /></blockquote>I was seeing this, from the Wikipedia definition---I wasn't getting the idea that precession **really** applied. But, for lack of another descriptive term, I tried to use it. Besides, we all (I think) understand that a spinning ball, while subject to gyroscopic precession--as **any** spinning body does--that it doesn't act like a top (or top-like object).

Regarding precession (http://en.wikipedia.org/wiki/Precession):
<blockquote><font class="small">Quote Fred:</font><hr>This definition again is 100% consistent with what I've said all along.<hr /></blockquote>No one was saying it wasn't, I included the link for reference purposes.

<blockquote><font class="small">Quote h451:</font><hr>sidespin and it's effect on swerve last year some time,<hr /></blockquote> <blockquote><font class="small">Quote Fred:</font><hr>There is no effect if the cue is level, which of course is hard to do. But if the cue was level, there would be no swerve due to the english. Even with, say, low-left or high-right.<hr /></blockquote>I'm still not sold on this--although, what I think may be incorrect, due to my missing of certain other variables. Either way, this still reads like an opinion. . . .that is, I myself do not have the data, and understanding of it, to believe it (yet). I will probably have to sort through some of Dr.Dave's work.

<blockquote><font class="small">Quote Fred:</font><hr> As far as I can tell, the cueball axis does not precess without interaction with any other forces.

Fred <hr /></blockquote>Does the inertia of the spin not count as a force--or does the force have to be "active", such as in a collision? I mean, I get how gravity can't pull the axis down, as it would with a top, but I would think something would still work on it.

===========================

BillPorter
08-16-2005, 05:03 PM
It amuses me to think of the reaction of the average pool player (or even a pro pool player) if they were forced to read this entire thread as their introduction to the CCB. What percentage of them would decide to visit again? Hey, I'm not being critical here, just amused.

nhp
08-17-2005, 06:14 AM
[ QUOTE ]
It amuses me to think of the reaction of the average pool player (or even a pro pool player) if they were forced to read this entire thread as their introduction to the CCB. What percentage of them would decide to visit again? Hey, I'm not being critical here, just amused. <hr /></blockquote>

Jal
08-17-2005, 04:30 PM
<blockquote><font class="small">Quote nhp:</font><hr> &lt;/font&gt;&lt;blockquote&gt;&lt;font class="small"&gt;Quote:&lt;/font&gt;&lt;hr /&gt;
It amuses me to think of the reaction of the average pool player (or even a pro pool player) if they were forced to read this entire thread as their introduction to the CCB. What percentage of them would decide to visit again? Hey, I'm not being critical here, just amused. <hr /></blockquote>

I don't know why you and some others who aren't interested in the scientific stuff feel compelled to proclaim it. There are a lot of pool related topics that hold little fascination for me, but I don't see what purpose would be served by intruding on those threads to say so. Maybe others do this -- I don't know, I don't read them.

Physics isn't going to be of much use to apparently experienced players like yourself. You may learn a tidbit now and then. Maybe, maybe not. But the general principles are good guidelines for the relatively new player, or the one that doesn't get to play for hours every day, or anyone who's just curious. So what's wrong with discussing them? There are a lot of people out there who are not seasoned veterans, after all, and I think it's far more interesting than simply advising "go shoot it a thousand times".

Suppose you had an upcoming tournament and it was announced that the laws of physics had been tweaked a little. The cueball would not be coming off the object ball in anything like what we've come to expect. And a description of this new phenomena was provided here. You could: a) get to a table and start practicing like hell, or b) read up on it first to avoid any possible confusion, and then get to a table and start practicing like hell. I think most people would choose the to do the latter if they were pressed for time. It could and probably would speed up the learning process.

But even if these discussions are only to satisfy some curiosity, so what? What great purpose does shooting pool serve?

Jim

heater451
08-17-2005, 04:44 PM

===========================

Jal
08-17-2005, 06:32 PM
<blockquote><font class="small">Quote heater451:</font><hr>
<blockquote><font class="small">Quote Fred:</font><hr> As far as I can tell, the cueball axis does not precess without interaction with any other forces.

Fred <hr /></blockquote>Does the inertia of the spin not count as a force--or does the force have to be "active", such as in a collision? I mean, I get how gravity can't pull the axis down, as it would with a top, but I would think something would still work on it...<hr /></blockquote>

When the cueball is in sliding frictional contact with something else at one point on its surface (ie, with the table bed), then the frictional force and the torque it generates will remain fixed in one direction until the ball is no longer sliding (ie, rolling). This will induce a spin (or a new spin component if it already has some spin) in the direction of the torque. If it already has some spin, the new component will add (vectorally) with the existing component and the result will be a shift in the direction of the original spin (unless the torque is acting in the direction of the existing spin).

When the ball is in frictional contact at two or more points on its surface (ie, table bed and another ball or cushion) the direction of the fricional forces and the combined torque they produce will, in general, vary while the contact is taking place. This is the closest thing to what is normally described as precession, but I don't think it quite fits the bill.

Technically, the spin itself has no "inertia" in that it does not resist any torque applied to the object that is spinning. But it sort of seems that way in that the new spin created by the torque gets added to it (vectorally) and the result can be surprising (movement at right angles to the force producing the torque). It is also no "force", but it does give rise to so-called "false forces" such as the Coriolis effect and centrifugal forces.

As has been mentioned, thinking about the individual spin components rather than what is happening to the combined spin tends to make things clearer, I think.

Hope this helps a little.

Jim

nhp
08-18-2005, 02:18 AM
I'm not complaining about the threads on physics, I was simply pointing out why this forum seems to have died down.

Colin
08-21-2005, 12:53 AM
<blockquote><font class="small">Quote Fred Agnir:</font><hr>
We (the science guys) often state that a cueball will not swerve if a level stroke is used. Furthermore, we state that the path of the cueball is not affected by side spin unless the cueball contacts a rail.

Fred &lt;~~~ could be all wet <hr /></blockquote>
I'm a bit late finding this thread but I wanted to give my reply to the original question posed by Fred.

It may help some to decipher my comments by reading my article on 'How the Nap Works' http://www.top147.com/magazine/2004007/2004007091316505557.htm

Firstly I believe there is some turn effected on the cue ball with side on a napless cloth, though it is usually quite small and insignificant compared to the swerve produced due to the elevation on of the cue.

The Cause!
I believe the cloth offers a small component of resistance above the absolute bottom point of the cue ball, and the leading side encounters more of this force that the trailing side of the ball. This leads to a slight precession and small amount of turn.

Proof:
As it is hard to actually see this on a napless cloth because of the effect of swerve from the elevated cue, the best evidence I have is from my experience on snooker nap cloth tables.

Clockwise spin down the nap turns the cue ball left, to a higher degree than closkwise turns the cue ball right when shooting against the nap.

The difference may be partly due to the swerve assisting, but having experimented quite a bit in play and with finger spinning I am quite certain that the above is the case.

So there is some additional friction at the leading edge of travel of the ball I believe, though it is quite slight.

An interesting aside is the 'magnus effect' that we see in soccer, tennis and golf, where clockwise spin turns the balls left due to the difference in air pressure created on the two sides of the ball.

I've never noticed the magnus effect on the cue ball, I expect because they are too heavy and smooth, but on hard shots it may play some factor in negating the swerve effect.

Bob_Jewett
08-21-2005, 09:22 PM
<blockquote><font class="small">Quote Colin:</font><hr>...Proof:
As it is hard to actually see this on a napless cloth because of the effect of swerve from the elevated cue, the best evidence I have is from my experience on snooker nap cloth tables.... <hr /></blockquote>
Here is a way: make a ball spin in place like a top. Use a stripe so you can see the spin. Hit the spinning ball with another ball just hard enough to get the spinning ball to the far end of the table. Watch the ball carefully to see if it curves on the way to the far cushion. Shoot the same shot with the other side spin along the same path. Report your findings.

Colin
08-21-2005, 11:26 PM
<blockquote><font class="small">Quote Bob_Jewett:</font><hr> <blockquote><font class="small">Quote Colin:</font><hr>...Proof:
As it is hard to actually see this on a napless cloth because of the effect of swerve from the elevated cue, the best evidence I have is from my experience on snooker nap cloth tables.... <hr /></blockquote>
Here is a way: make a ball spin in place like a top. Use a stripe so you can see the spin. Hit the spinning ball with another ball just hard enough to get the spinning ball to the far end of the table. Watch the ball carefully to see if it curves on the way to the far cushion. Shoot the same shot with the other side spin along the same path. Report your findings. <hr /></blockquote>

Hi Bob,
What you describe is basically what I used to do to analyze the spin effects on a snooker nap cloth. But I'd spin the ball by hand and then bump it with another ball in the appropriate direction.

It appears that the down the nap turn was greater than the up the nap turn.

The conclusion is that there is a factor of higher leading edge resistance, as the leading edge resitance works against the nap turn we see moving against the nap.

What have similar experiments shown on napless cloths? Any noticeable turn? I expect the rougher cloths would show some sign of turn.

Bob_Jewett
08-22-2005, 10:54 AM
<blockquote><font class="small">Quote Colin:</font><hr> ... What have similar experiments shown on napless cloths? Any noticeable turn? I expect the rougher cloths would show some sign of turn. <hr /></blockquote>
I have not seen any detectable curve on cloths like Simonis 860.

Colin
08-22-2005, 11:16 AM
From the few times I've played around on napless cloths, I haven't noticed any turn either so far.

I'll be curious to try it on some thicker rougher napless cloths though.

dr_dave
08-22-2005, 12:11 PM
<blockquote><font class="small">Quote Colin:</font><hr> From the few times I've played around on napless cloths, I haven't noticed any turn either so far.

I'll be curious to try it on some thicker rougher napless cloths though.<hr /></blockquote>
I have a table at home with a cloth that meets that description, and I also cannot detect any noticeable turn due to rolling sidespin.

Dave

BoroNut
08-23-2005, 11:10 AM
<blockquote><font class="small">Quote wolfdancer:</font><hr> .....or any dog catching a Frisbee. We could throw in some aerodynamics for the dog.<hr /></blockquote>

Not sure that's entirely necessary. As long as the frisbee's aerodynamic it doesn't seem to matter what shape the dog is.

Boro Nut

Jal
08-24-2005, 01:29 PM
<blockquote><font class="small">Quote Colin:</font><hr>...
Firstly I believe there is some turn effected on the cue ball with side on a napless cloth, though it is usually quite small and insignificant compared to the swerve produced due to the elevation on of the cue.

The Cause!
I believe the cloth offers a small component of resistance above the absolute bottom point of the cue ball, and the leading side encounters more of this force that the trailing side of the ball. This leads to a slight precession and small amount of turn...<hr /></blockquote>

There is a kind of self-correcting or negative feedback mechanism at work here.

Suppose you apply right-english with a perfectly level cue and the leading vs trailing edge frictions result in a net force to the right. This will produce a torque and a spin component pointing back at you. When this new spin component is added to the existing one, which is pointing straight up, the spin axis will tilt back a little (ignoring the other tilt due to the developing follow). The effect will be to at least partially neutralize the force pushing the ball to the right. Whether it's significant or not, I don't know.

The curves of the balls when shooting against the nap as shown at your site, suggest it isn't. Although these are cases where the trailing edge friction is greater than the leading edge friction, the mechanism should be the same.

Just more froth.

Jim